another response from Denny:
The term nominal, is an interesting conversation on its own. Therefore Iím not real sure your definition, but I will use the information contained in the ASTM E-11 specification for test sieves.
A couple of other facts about the MIL-PFR9854B specification.
1. U.S. Standard screen number is the alternate sieve number in ASTM E-11, and at one time was refer to the mesh of the sieve or the number of opening per inch of a sieve.
2. In table one the % passing requirement is reversed compared to many shot peening sieve specifications. 100% passing would be stated in many other peening specifications as 0% max.
Using table 1, the nominal size for the size 3 glass would be the, 30 US STD screen, which is the ASTM No 30, 600 micron, .0234 inch nominal opening test sieve. The sieve would be required to have 85% minimum retained on it or no more than 15% passing.